Illuminations: Anywhere around WSC lagoon with a clear shot of the fountains and the fire barge. If you can see them clearly, you got yourself a perfect spot. Other places with great views that fill up early: the French Island*, Italy's Isola*, Mexico's bridge, Canada's rocks, and the entrance of WSC.
*These locations may be closed due to conventions.
Fantasmic: I'd say show up about an hour early to guarantee yourself a decent seat. I believe they fill up the center of the theater first then work thier way out.